The use of x-ray computed tomography (CT)-based attenuation correction for positron emission tomography (PET) in PET/CT systems requires the transformation of CT Hounsfield units (HU) to linear attenuation coefficients at 511 keV (LAC511). This cannot be done perfectly from a single peak kilovolt (kVp) CT scan due to variability in Compton and photoelectric composition and, thus, an approximate transformation must be employed. One difficulty in this lies in accurately determining the linear attenuation coefficients (LAC) in actual human tissue. Typically, phantoms consisting of synthetic materials thought to be approximate human tissue equivalents are employed instead. A potentially more accurate approach would be to use dual kVp CT scans to estimate LAC511 in actual human tissue and then base the single kVp transformation on these data. This approach would also permit an assessment of the dispersion of actual tissue values about the two-component trend lines typically used for the single kVp transformation. In this paper, we develop and assess this methodology.